System and method for diagnosing the selective catalytic reduction system of a motor vehicle

ABSTRACT

A system and method for diagnosing selective catalytic reduction system of a motor vehicle, the system including an internal combustion engine connected by an exhaust manifold to, successively, an assembly including a nitrogen oxide catalyst and a particle filter and a selective catalytic reduction system, a mechanism draining mass of ammonia stored in the selective catalytic reduction system, a mechanism controlling draining, and a mechanism injecting urea positioned upstream of the selective catalytic reduction system, a mechanism measuring quantity of ammonia at an outlet of the selective catalytic reduction system, a system determining maximum mass of ammonia stored in the selective catalytic reduction system, and a comparison mechanism configured to transmit a fault signal based on a result of a comparison between the maximum mass of ammonia stored in the selective catalytic reduction system and a threshold.

The area of the invention is the on-board diagnosis of functions of amotor vehicle, more particularly the diagnosis of the catalyticreduction of polluting emissions of a diesel-type motor vehicle.

In order to respond to the lower limits for the permitted emission ofpollutants from motor vehicles, increasingly complex exhaust gasafter-treatment systems are provided in the exhaust tract of lean-burnengines. These allow reduction in particular of the emissions ofparticulates and nitrous oxides, as well as carbon monoxide and unburnthydrocarbons.

The selective catalytic reduction system (SCR) is known as an effectivemeans for treatment for nitrous oxides (NO_(X)). The system comprisescontinuous treatment of nitrous oxide emissions (nitrates and nitrites).It requires the use of a catalyst and a reducing agent injector arrangedin the exhaust system.

The system therefore requires the addition of an additional tankcontaining the reducing agent (AdBlue for example), the specificinjection system, a system for mixing the reducing agent with theexhaust gases, and a catalyst system accelerating the reduction ofnitrous oxides by the reducing agent injected and/or stored on thecatalyst. It is noted that, in order to optimize the efficiency ofnitrous oxide treatment, the mixture entering the catalyst of theselective catalytic reduction system SCR must be as homogenous aspossible, which requires the use of the abovementioned mixing system.

In this system, the quantity of reducing agent injected and the quantityof reducing agent stored on the catalyst must be finely adapted: infact, an overdose of reducing agent (stored or injected) would onlypointlessly increase the consumption of reducing agent and perhapsgenerate emissions of ammonia at the exhaust (a highly odorous andhighly toxic compound). Under-dosing however limits the efficiencyobtained and hence increases the emissions of nitrous oxides at theexhaust.

Chemically, the selective catalytic reduction system stores the ammonia(NH₃) contained in the reducing agent, such as urea. The ammonia thusstored in the selective catalytic reduction system then reduces thenitrous oxides (NO_(x)). For optimum efficiency of the selectivecatalytic reduction system, it is necessary to control the stored massof ammonia (NH₃), also called the ammonia buffer (NH₃). However, as theselective catalytic reduction system ages, its ammonia storage capacity(NH₃) degrades, leading to a loss of efficiency.

Euro 5 and Euro 6 standards impose an on-board diagnosis (OBD) systemfor the entire selective catalytic reduction system. This thereforerequires recurrent verification that the selective catalytic reductionsystem remains sufficiently effective at treating nitrous oxides(NO_(x)) to meet the on-board diagnosis thresholds. Until now, diagnosisof the selective catalytic reduction system SCR has been based on thedegradation of efficiency of the treatment of nitrous oxides (NO_(x)).

However, it is not possible at present to diagnose the loss of storagecapacity for the ammonia (NH₃) which is a more direct consequence ofageing. The main difficulty of the new diagnostic method is that it mustbe able to discriminate between a reversible efficiency loss, which iscompensated for example by adapting the control of the urea injection,from a definitive loss when the SCR no longer has any storage sites forammonia NH₃ for treating the nitrous oxides NO_(x). This requiresdiagnosis of the ammonia storage capacity (ASC), a value characterizedby a maximum mass of ammonia (NH₃) which can be stored, which is adirect reflection of the ageing state of the SCR.

From the prior art, the following documents are known.

Document WO 2008/103113 discloses an on-board diagnosis (OBD) of the SCRsystem based on the efficiency of the treatment of nitrous oxides NO_(x)under specific conditions. The efficiency of treatment of the nitrousoxides NO_(x) by the selective catalytic reduction system SCR greatlydepends on the NO₂/NO_(x) ratio between the nitrogen dioxide and thenitrous oxides at the inlet to the selective catalytic reduction systemSCR. This ratio cannot be measured and is affected by the sulphurizationlevel of the catalyst DOC, and by the soot level of the particulatefilter FAP upstream of the selective catalytic reduction system SCR. Theefficiency determined by the model of the on-board selective catalyticreduction system SCR is compared with that determined via a sensor fornitrous oxides NO_(x) arranged downstream of the selective catalyticreduction system SCR, under conditions where the NO₂/NO_(x) ratio is apriori nominal, i.e. after desulphurization of the catalyst DOC and/orregeneration of the particulate filter FAP. If, under nominalconditions, the efficiency measured is less than the efficiencycalculated by the model, a failure of the selective catalytic reductionsystem SCR is detected.

Document WO 2007/037730 discloses a diagnosis system for the selectivecatalytic reduction system SCR based on comparison of the efficiency ofthe treatment of the nitrous oxides NO_(x) by a nitrous oxides NO_(x)sensor arranged downstream of the selective catalytic reduction systemSCR, with a nominal efficiency at a given engine operating point (load,speed). If the efficiency of the SCR is lower than the reference, afailure is detected.

There is therefore a need for a system and a method for diagnosis of theselective catalytic reduction system taking into account the loss ofammonia storage capacity, a direct consequence of ageing.

An object of the invention is a system for diagnosing the selectivecatalytic reduction system of a motor vehicle equipped with an internalcombustion engine connected via an exhaust manifold successively to anassembly comprising a nitrous oxide catalyst and a particulate filter,and a selective catalytic reduction system. The system comprises:

-   -   a means for evacuation of the ammonia mass stored in the        selective catalytic reduction system,    -   a urea injection means arranged upstream of the selective        catalytic reduction system,    -   a means for control of the evacuation and injection,    -   a means for measuring the quantity of ammonia at the outlet from        the selective catalytic reduction system,    -   a system for determining the maximum mass of ammonia stored in        the selective catalytic reduction system,    -   a means for comparing the value determined for the maximum mass        of ammonia stored in the selective catalytic reduction system        with a threshold, and able to emit a fault signal as a function        of the comparison result.

The means for determining the maximum mass of ammonia stored in theselective catalytic reduction system may comprise a means fordetermining the ammonia level at the outlet from the selective catalyticreduction system as a function of the signal received from the measuringmeans, and a means for modeling the selective catalytic reductionsystem, and able to estimate the levels of ammonia and nitrous oxidesdownstream of the selective catalytic reduction system via a model. Themeans for determining the maximum mass may also comprise a calculationmeans able to determine the difference between the level of ammoniameasured at the outlet from the selective catalytic reduction system andthe modeled values for the levels of ammonia and nitrous oxidesdownstream of the selective catalytic reduction system, and to determinea new value for the maximum mass of ammonia stored in the selectivecatalytic reduction system if the difference is positive, and totransmit the new value for the maximum mass of ammonia stored in theselective catalytic reduction system to the means for modeling theselective catalytic reduction system in order to determine new valuesuntil the difference is zero. The calculation means may be able totransmit the maximum mass of ammonia stored in the selective catalyticreduction system when the difference is zero.

The fault signal for the selective catalytic reduction system may assumea first value if the value determined for the maximum mass of ammoniastored in the selective catalytic reduction system is less than athreshold, while it assumes a second value if this is not the case.

The means for modeling the selective catalytic reduction system may beable to estimate the ammonia level downstream of the selective catalyticreduction via a model, as a function of the ratio between the nitrogendioxide and the nitrous oxides downstream of the exhaust manifold, thetemperature upstream of the selective catalytic reduction system, themass of urea injected upstream of the selective catalytic reductionsystem, the maximum mass of ammonia stored in the selective catalyticreduction system, the exhaust gas flow, the level of ammonia upstream ofthe selective catalytic reduction system, and the level of nitrousoxides upstream of the selective catalytic reduction system.

The means for modeling the selective catalytic reduction system may alsobe able to determine the ratio between nitrogen dioxide and nitrogenmonoxide downstream of the exhaust manifold, as a function of thetemperature upstream of the assembly of the particulate filter and thecatalyst, the exhaust gas flow from the internal combustion engine, andthe level of nitrous oxides downstream of the exhaust manifold.

Another object of the invention is a method for diagnosing the selectivecatalytic reduction system of a motor vehicle equipped with an internalcombustion engine connected via an exhaust manifold successively to anassembly comprising a nitrous oxide catalyst and a particulate filter,and a selective catalytic reduction system. The method comprises thefollowing steps:

-   -   evacuation of the ammonia mass stored in the selective catalytic        reduction system,    -   injection, upstream of the selective catalytic reduction system,        of a mass of urea greater than the maximum mass which can be        stored by the selective catalytic reduction system,    -   determination of the maximum mass of ammonia stored in the        selective catalytic reduction system,    -   emission of a fault signal as a function of the result of        comparison of the value determined for the maximum mass of        ammonia stored in the selective catalytic reduction system with        a threshold.

The maximum mass of ammonia stored in the selective catalytic reductionsystem may be determined by performing the following steps:

-   -   initialization of a model with a saved value for the maximum        mass which can be stored in the selective catalytic reduction        system,    -   estimation of the levels of ammonia and nitrous oxides        downstream of the selective catalytic reduction system via a        model,    -   measurement of the ammonia level at the outlet from the        selective catalytic reduction system,    -   determination of the difference between the ammonia level at the        outlet from the selective catalytic reduction system and the        modeled values for the levels of ammonia and nitrous oxides        downstream of the selective catalytic reduction system,    -   if the difference is positive, determination of a new value for        the maximum mass of ammonia stored in the selective catalytic        reduction system, and determination of new values for the levels        of ammonia and nitrous oxides downstream of the selective        catalytic reduction system via the model until the difference is        zero,    -   when the difference is zero, the value of the maximum mass        stored in the selective catalytic reduction system is emitted.

If the value determined for the maximum mass of ammonia stored in theselective catalytic reduction system is less than a threshold, a faultsignal assuming a first value may be emitted for the selective catalyticreduction system, while it assumes a second value if this is not thecase.

The levels of ammonia and nitrous oxides downstream of the selectivecatalytic reduction system may be estimated via a model, as a functionof the ratio between the quantity of nitrogen monoxide and nitrogendioxide downstream of the exhaust manifold, the temperature upstream ofthe selective catalytic reduction system, the mass of urea injectedupstream of the selective catalytic reduction system, the maximum massof ammonia stored in the selective catalytic reduction system, theexhaust gas flow, the level of ammonia upstream of the selectivecatalytic reduction system, and the level of nitrous oxides upstream ofthe selective catalytic reduction system.

The ratio between the nitrogen monoxide and the nitrogen dioxidedownstream of the exhaust manifold may be determined as a function ofthe temperature upstream of the assembly of the particulate filter andthe catalyst, the exhaust gas flow from the internal combustion engine,and the level of nitrous oxides downstream of the exhaust manifold.

Further aims, characteristics and advantages will appear from readingthe description below, given solely as a non-limitative example, andwith reference to the attached drawings on which:

FIG. 1 shows the principal elements of an internal combustion engineequipped with a selective catalytic reduction system and an on-boarddiagnostic system,

FIG. 2 illustrates the main elements of a system for determining themaximum ammonia storage capacity, and

FIG. 3 illustrates the main steps of the method for diagnosing theselective catalytic reduction system.

FIG. 1 illustrates an internal combustion engine 1 of a motor vehicleconnected by its exhaust manifold to an exhaust pipe. Mounted on theexhaust pipe downstream of the exhaust manifold, we see successively afirst temperature sensor 2, an assembly 3 comprising an oxidationcatalyst and a particulate filter, a urea injector 4, a secondtemperature sensor 5, a selective catalytic reduction system 6 and anitrous oxide sensor 7.

Preferably, the oxidation catalyst is housed in the assembly 3 upstreamof the particulate filter, i.e. closer to the engine 1, so as to reachits ignition temperature more quickly.

Also preferably, the second temperature sensor 5 is installed upstreamof the injector 4, such that the temperature measurement is notdisrupted by the urea injection.

The internal combustion engine 1 is connected directly or via a controlmeans to an on-board diagnostic system 8 via a connection providing theexhaust gas flow and the flow of nitrous oxides NO_(x). These flowsresult from a map or from an estimation means, depending in particularon the operating point of the internal combustion engine 1.

The first temperature sensor 2 is connected to the on-board diagnosticsystem 8 via a connection 2 a providing the temperature upstream of theassembly 3.

The second temperature sensor 5 is connected to the on-board diagnosticsystem 8 via a connection 5 a providing the temperature upstream of theselective catalytic reduction system 6.

The nitrous oxides sensor 7 is connected to the on-board diagnosticsystem 8 via a connection 7 a providing the quantity of nitrous oxidesand ammonia downstream of the selective catalytic reduction system 6.

To diagnose the selective catalytic reduction system (SCR), the maximummass of ammonia stored is estimated under conditions of leakage of thisammonia. The nitrous oxides (NO_(x)) sensor, reference 7, arrangeddownstream of the selective catalytic reduction system 6, cannotdistinguish nitrous oxides (NO_(x)) from ammonia (NH₃). This propertycan then be exploited to detect leaks of ammonia (NH₃), deduce from thisthe maximum mass of ammonia stored, and diagnose a failure as a functionof this value.

However, to estimate the maximum mass of ammonia stored, it is necessaryto estimate the mass of ammonia stored at a given instant as a functionof a model of the selective catalytic reduction system 6. The modeldescribed below is included in the system 9 for determining the maximumammonia storage capacity. To be able to estimate the maximum mass ofammonia stored, the model is linked to the nitrous oxides NO_(x) sensor,reference 7.

Also, to determine the maximum mass of ammonia stored, the selectivecatalytic reduction system is evacuated via a control means 8 a for theevacuation and injection of urea, linked to an actuator via connection 6a. Alternatively, the control means 8 a may interrupt the urea injectionby the injector 4 in order to obtain an effect equivalent to evacuationby consumption of all the ammonia present in the selective catalyticreduction system. Since evacuation allows an absolute reference value tobe set, the mass determined does not comprise any relativity and cantherefore be compared with a threshold by a comparison means 8 b inorder to determine a failure of the selective catalytic reductionsystem.

The description presented below firstly comprises presentation of themodeling of the selective catalytic reduction system 6, then the methodand system of diagnosing the selective catalytic reduction system.

FIG. 2 illustrates a system 9 for determining the maximum ammoniastorage capacity. This has a means 9 a for determining the ammonia levelat the outlet from the selective catalytic reduction system 6, as afunction of the measurement by the nitrous oxides sensor 7.

Also there is a means 9 b for modeling the selective catalytic reductionsystem. The modeling of the selective catalytic reduction system beginsby estimating the ammonia mass. This is given by a reduced model basedon the physico-chemical phenomena taking place in the exhaust tract.

The ratio α between the nitrogen dioxide (NO₂) and the nitrous oxides(NO_(x)), imposed by the combination of the oxidation catalyst (DOC) andthe particulate filter (FAP) at the inlet to the selective catalyticreduction system 6, has a great influence on the efficiency of theselective catalytic reduction system 6.

However, this ratio α between the nitrogen dioxide (NO₂) and the nitrousoxides (NO_(x)) cannot be measured. It must therefore be estimated via amodel or map as a function of the exhaust gas flow q_(ech) and thetemperature at the inlet to the oxidation catalyst (DOC), calledT_(doc). The exhaust flow q_(ech) is measured or modeled as a functionof the different engine gas flows.

We can then determine the level of nitrogen monoxide X^(in) _(no) andnitrogen dioxide X^(in) _(no2) at the inlet to the selective catalyticreduction system as a function of the level of nitrous oxides (NO_(x))at the outlet from the engine X^(in) _(nox) and the ratio between thenitrogen dioxide (NO₂) and the nitrous oxides (NO_(x)), called α. Thefollowing equation explains this determination.

X _(no2) ^(in)=α(T _(doc) ,q _(ech))·X _(nox) ^(in)

X _(no) ^(in)=(1−α(T _(doc) ,q _(ech)))·X _(nox) ^(in)  (Eq. 1)

It is noted that the level of nitrous oxides (NO_(x)) at the outlet fromthe engine X^(in) _(nox) can be measured via a sensor or estimated via amodel.

The simplified reactional mechanism below reflects the function of theselective catalytic reduction system:

NH₃+*→NH₃*+ . . .

NH₃*→NH₃+*

NO+NH₃*→N₂+ . . .

NO+NO₂+2NH₃*→2N₂+ . . .

NO₂+NH₃*→N₂+ . . .

NH₃+O₂→N₂+ . . .  (Eq. 2)

wherein:*: is a site which can receive a molecule of ammonia NH₃NH₃*: represents a stored molecule of ammonia NH₃O₂: a molecule of dioxygenN₂: a molecule of dinitrogenNO: a molecule of nitrogen monoxideNO₂: a molecule of nitrogen dioxide.

It is possible to model the reactional mechanism described below inorder to obtain dynamically the level X^(out) _(nox) of nitrous oxides(NO_(x)) and the level X^(out) _(nh3) of ammonia (NH₃) downstream of theselective catalytic reduction system, and the stored mass m_(nh3) (alsocalled the buffer) of ammonia (NH₃) as a function of the level X^(in)_(nox) of nitrous oxides (NO_(x)) and the level X^(in) _(nh3) of ammonia(NH₃) upstream of the selective catalytic reduction system, the exhaustgas flow q_(ech), the temperature T_(scr) of these gases at the inlet tothe selective catalytic reduction system, and finally the maximum massof ammonia stored in the selective catalytic reduction system, calledm^(max) _(nh3).

The equation system illustrates such a model.

$\begin{matrix}{{\frac{m_{{nh}\; 3}}{t} = {f\left( {X_{nox}^{in},X_{{nh}\; 3}^{in},m_{{nh}\; 3},T_{scr},q_{ech},\alpha,m_{{nh}\; 3}^{\max}} \right)}}{X_{nox}^{out} = {h_{nox}\left( {X_{nox}^{in},X_{{nh}\; 3}^{in},m_{{nh}\; 3},T_{scr},q_{ech},\alpha,m_{{nh}\; 3}^{\max}} \right)}}{X_{{nh}\; 3}^{out} = {h_{{nh}\; 3}\left( {X_{nox}^{in},X_{{nh}\; 3}^{in},m_{{nh}\; 3},T_{scr},q_{ech},\alpha,m_{{nh}\; 3}^{\max}} \right)}}} & \left( {{Eq}.\mspace{11mu} 3} \right)\end{matrix}$

The level X^(in) _(nh3) of ammonia (NH₃) upstream of the selectivecatalytic reduction system can be estimated as a function of theinjected urea quantity.

With reference still to FIG. 2, we see that the means 9 b for modelingthe selective catalytic reduction system determines the variation inmass dm_(nh3)/dt of ammonia (NH₃) as a function of time, the levelX^(out) _(nox) of nitrous oxides (NO_(x)) and the level X^(out) _(nh3)of ammonia (NH₃) downstream of the selective catalytic reduction system,as a function of the level X^(in) _(nox) of nitrous oxides (NO_(x))upstream of the selective catalytic reduction system and the flowq_(ech) of exhaust gas from the internal combustion engine 1, thetemperature T_(scr) of these gases at the inlet to the selectivecatalytic reduction system from the second temperature sensor 5, theratio α between the nitrogen dioxide (NO₂) and the nitrous oxides(NO_(x)), the maximum mass m^(max) _(nh3) of ammonia stored in theselective catalytic reduction system, the level X^(in) _(nh3) of ammonia(NH₃) upstream of the selective catalytic reduction system, and thecurrent mass m_(nh3) of ammonia stored in the selective catalyticreduction system. It is noted that the maximum mass m^(max) _(nh3) ofammonia stored in the selective catalytic reduction system is a datumfor design of the selective catalytic reduction system 6.

By knowing the variation in mass of ammonia over time dm_(nh3)/dt, thelevel X^(out) _(nox) of nitrous oxides (NO_(x)) and the level X^(out)_(nh3) of ammonia (NH₃) downstream of the selective catalytic reductionsystem, it is possible to determine the maximum mass of ammonia storedvia an observer. Prior to definition of the observer, the measurementfrom the nitrous oxides sensor (NO_(x)) downstream of the selectivecatalytic reduction system is broken down to take account of itscapacity to measure both nitrous oxides and ammonia. The followingequation takes into account this breakdown.

X _(nox) ^(out,capt) =X _(nox) ^(out) +X _(nh3) ^(out)  (Eq. 4)

wherein:X^(out,capt) _(nox): value of the measurement of the nitrous oxidessensorX^(out) _(nox): level of nitrous oxidesX^(out) _(nh3): level of ammonia (NH₃) downstream of the selectivecatalytic reduction system.

This distribution between two contributions, as has been describedabove, reflects the fact that it is not possible to distinguish betweennitrous oxides (NO_(x)) and ammonia (NH₃) at the selective catalyticreduction system.

By combining equations 3 and 4, we propose the following observer:

$\begin{matrix}{{\frac{m_{{nh}\; 3}}{t} = {f\left( {X_{nox}^{in},X_{{nh}\; 3}^{in},m_{{nh}\; 3},T_{scr},q_{ech},\alpha,m_{{nh}\; 3}^{\max}} \right)}}{X_{nox}^{out} = {h_{nox}\left( {X_{nox}^{in},X_{{nh}\; 3}^{in},m_{{nh}\; 3},T_{scr},q_{ech},\alpha,m_{{nh}\; 3}^{\max}} \right)}}{X_{{nh}\; 3}^{out} = {h_{{nh}\; 3}\left( {X_{nox}^{in},X_{{nh}\; 3}^{in},m_{{nh}\; 3},T_{scr},q_{ech},\alpha,m_{{nh}\; 3}^{\max}} \right)}}{\frac{m_{{nh}\; 3}^{\max}}{t} = {K\; \Delta}}} & \left( {{Eq}.\mspace{11mu} 5} \right)\end{matrix}$

wherein:

Δ=X _(nox) ^(out,capt) −X _(nox) ^(out) −X _(nh3) ^(out)  (Eq. 6)

K: strictly positive gain

On FIG. 2, we can see that the system 9 for determining the maximumammonia storage capacity also comprises a subtractor 9 c connected atthe inlet to the means 9 a for determining the ammonia level at theoutlet from the selective catalytic reduction system 6, and to the means9 b for modeling the selective catalytic reduction system, and connectedat the outlet to a calculation means 9 d able to determine the maximummass m^(max) _(nh3) of ammonia stored in the selective catalyticreduction system as a function of the signal received from thesubtractor 9 c and from a memory 9 e.

The subtractor 9 c allows determination of the parameter Δ byapplication of equation 6, by subtracting the values received from themeans 9 b for modeling the selective catalytic reduction system from thevalue received from the determination means 9 a.

The calculation means 9 d determines the maximum mass m^(max) _(nh3) ofammonia stored in the selective catalytic reduction system as a functionof the signal received from the memory 9 e when the system 9 fordetermining the maximum ammonia storage capacity is initialized. Inother situations, the calculation means 9 d determines the maximum massm^(max) _(nh3) of ammonia stored in the selective catalytic reductionsystem by integrating, relative to time, the product of parameter Δ bythe saved constant K. In doing this, the calculation means 9 d appliesthe third equation of the equation system (Eq 5).

The calculation means 9 d also estimates whether the parameter Δ iszero. If so, the calculation means 9 d emits the determined value forthe maximum mass m^(max) _(nh3) of ammonia stored in the selectivecatalytic reduction system.

The comparison means 8 b receives from the calculation means 9 d asignal carrying the maximum mass m^(max) _(nh3) of ammonia stored in theselective catalytic reduction system 6. The comparison means 8 bperforms the comparison of the maximum mass with a memorized threshold,allowing a distinction between a selective catalytic reduction system ingood condition and a faulty selective catalytic reduction system. If themaximum mass is greater than the threshold, a signal for absence offailure is transmitted, otherwise a failure signal is transmitted.Alternatively, only the failure signal is emitted, and only when theemission conditions are combined.

Use of the observer described above will be better understood fromreading the numerical example below.

Let us assume that the maximum mass of ammonia which can be stored in aselective catalytic reduction system in good condition is equal to 4 g.

Also, as we are in leakage conditions for ammonia (NH₃), we can ignorethe nitrous oxides (NO_(x)) at the outlet from the selective catalyticreduction system. In fact the nitrous oxides will be reduced by theexcess of ammonia (NH₃).

The model is initialized with a maximum m^(max) _(nh3) of ammonia storedin the catalyst equal to the value of the maximum mass of ammonia whichcan be stored in a selective catalytic reduction system in goodcondition, or 4 g in the present example, then the values for the levelof nitrous oxides and ammonia (NH₃) downstream of the selectivecatalytic reduction system are determined.

Then we determine the value of parameter Δ as a function of the modeledvalues for the levels of nitrous oxides and ammonia (NH₃) downstream ofthe selective catalytic reduction system, and of the measurement fromthe nitrous oxides sensor.

If parameter Δ is positive, the maximum mass m^(max) _(nh3) of ammoniastored in the catalyst is less than the maximum mass with which theselective catalytic reduction system was modeled.

We then determine a new maximum mass as a function of the parameter Δdetermined. The new maximum mass is re-injected at the inlet to themodel, such that we determine new values for the levels of nitrousoxides and ammonia (NH₃) downstream of the selective catalytic reductionsystem.

These new values are then compared with the measurement of the nitrousoxides level at the outlet from the selective catalytic reductionsystem, in order to determine a new value for parameter Δ. If parameterΔ is positive, the method continues by determining new values for themaximum mass m^(max) _(nh3) of ammonia stored in the catalyst, and thelevels of nitrous oxides and ammonia (NH₃) downstream of the selectivecatalytic reduction system, until we obtain a value of zero forparameter Δ.

As soon as parameter Δ is zero, the current value of the maximum massm^(max) _(nh3) of ammonia stored in the catalyst is saved and comparedwith a threshold. If the value is greater than the threshold, theselective catalytic reduction system is in good condition, otherwise afault is detected and a warning signal emitted.

The method for diagnosing the selective catalytic reduction systemillustrated by FIG. 3 uses the models and equations explained above.

The method comprises a first step 10 during which the ammonia mass isevacuated. In fact, in order to perform the diagnosis under the bestconditions, an evacuation is performed so that the estimate of themaximum mass m^(max) _(nh3) of ammonia stored in the selective catalyticreduction system can begin from an absolute reference substantiallyequal to zero. This evacuation takes place by cutting the urea injectionfor a few minutes.

During the second step 11, a specific mass of urea is injected which issufficiently high for the selective catalytic reduction system to eitherreach or exceed the limit for leakage of ammonia (NH₃). In other words,more ammonia (NH₃) is injected than the system can theoreticallycontain. The mass to be injected may be determined as a function of themaximum mass of ammonia stored in a catalytic reduction system having nofault.

During a third step 12, we estimate the ammonia level x^(out) _(nh3) andthe nitrous oxides level x^(out) _(nh3) downstream of the selectivecatalytic reduction system via the model by applying equation 5. We alsomeasure the level x^(out,capt) _(nox) of ammonia downstream of theselective catalytic reduction system via the nitrous oxides sensor 7. Wethen determine parameter Δ by applying equation 6. If parameter Δ ispositive, a new value is determined for the maximum mass m^(max) _(nh3)of ammonia stored in the selective catalytic reduction system as afunction of the value of the parameter Δ. New values are then determinedfor the level of ammonia x^(out) _(nh3) and nitrous oxides x^(out)_(nh3) downstream of the selective catalytic reduction system via themodel before determining a new value for parameter Δ.

The third step is repeated until the value of parameter Δ is zero. Thelast value of the maximum mass m^(max) _(nh3) of ammonia stored in theselective catalytic reduction system is then saved.

During a fourth step 13, we compare the maximum mass m^(max) _(nh3) ofammonia stored in the selective catalytic reduction with a threshold. Ifthe maximum mass is greater than the threshold, the selective catalyticreduction system has no fault. If this is not the case, the selectivecatalytic reduction system has a fault.

A signal corresponding to the state of the selective catalytic reductionsystem is then transmitted at the outlet.

1-10. (canceled) 11: A system for diagnosing selective catalyticreduction system of a motor vehicle including an internal combustionengine connected via an exhaust manifold successively to an assemblyincluding a nitrous oxide catalyst and a particulate filter, and aselective catalytic reduction system, the system comprising: a means forevacuation of ammonia mass stored in the selective catalytic reductionsystem; a urea injection means arranged upstream of the selectivecatalytic reduction system; a means for control of the evacuation andinjection; a means for measuring quantity of ammonia at an outlet fromthe selective catalytic reduction system; a system for determiningmaximum mass of ammonia stored in the selective catalytic reductionsystem; a means for comparing a value determined for the maximum mass ofammonia stored in the selective catalytic reduction system with athreshold, and to emit a fault signal as a function of the comparisonresult. 12: The system as claimed in claim 11, wherein the means fordetermining the maximum mass of ammonia stored in the selectivecatalytic reduction system comprises: a means for modeling the selectivecatalytic reduction system, and to estimate levels of ammonia andnitrous oxides downstream of the selective catalytic reduction systemvia a model; a means for determining a level of ammonia at the outletfrom the selective catalytic reduction system as a function of thesignal received from the measurement means; a calculation meansconfigured to determine a difference between the level of ammoniameasured at the outlet from the selective catalytic reduction system andmodeled values for levels of ammonia and nitrous oxides downstream ofthe selective catalytic reduction system, to determine a new value forthe maximum mass of ammonia stored in the selective catalytic reductionsystem if the difference is positive, and to transmit the new value forthe maximum mass of ammonia stored in the selective catalytic reductionsystem to the means for modeling the selective catalytic reductionsystem to determine new values until the difference is zero; thecalculation means configured to emit the maximum mass of ammonia storedin the selective catalytic reduction system when the difference is zero.13: The system as claimed in claim 11, wherein the fault signal for theselective catalytic reduction system assumes a first value if the valuedetermined for the maximum mass of ammonia stored in the selectivecatalytic reduction system is less than a threshold, while it assumes asecond value if that is not the case. 14: The system as claimed in claim12, wherein the means for modeling the selective catalytic reductionsystem is configured to estimate the ammonia level downstream of theselective catalytic reduction system via a model, as a function of theratio between the nitrogen dioxide and the nitrous oxides downstream ofthe exhaust manifold, temperature upstream of the selective catalyticreduction system, the mass of urea injected upstream of the selectivecatalytic reduction system, the maximum mass of ammonia stored in theselective catalytic reduction system, exhaust gas flow, the level ofammonia upstream of the selective catalytic reduction system, and thelevel of nitrous oxides upstream of the selective catalytic reductionsystem. 15: The system as claimed in claim 14, wherein the means formodeling the selective catalytic reduction system is configured todetermine the ratio between nitrogen dioxide and nitrogen monoxidedownstream of the exhaust manifold, as a function of temperatureupstream of the assembly of the particulate filter and the catalyst,exhaust gas flow of the internal combustion engine, and level of nitrousoxides downstream of the exhaust manifold. 16: A method for diagnosing aselective catalytic reduction system of a motor vehicle including aninternal combustion engine connected via an exhaust manifoldsuccessively to an assembly including a nitrous oxide catalyst and aparticulate filter, and a selective catalytic reduction system, themethod comprising: evacuation of ammonia mass stored in the selectivecatalytic reduction system; injection, upstream of the selectivecatalytic reduction system, of a mass of urea greater than a maximummass which can be stored by the selective catalytic reduction system;determination of the maximum mass of ammonia stored in the selectivecatalytic reduction system; and emission of a fault signal as a functionof a result of comparison of a value determined for the maximum mass ofammonia stored in the selective catalytic reduction system with athreshold. 17: The method as claimed in claim 16, wherein the maximummass of ammonia stored in the selective catalytic reduction system isdetermined by performing: initialization of a model with a saved valuefor the maximum mass which can be stored in the selective catalyticreduction system; estimation of levels of ammonia and nitrous oxidesdownstream of the selective catalytic reduction system via a model;measurement of an ammonia level at the outlet from the selectivecatalytic reduction system; determination of a difference between theammonia level at the outlet from the selective catalytic reductionsystem and the modeled values for the levels of ammonia and nitrousoxides downstream of the selective catalytic reduction system; and ifthe difference is positive, determination of a new value for the maximummass of ammonia stored in the selective catalytic reduction system, anddetermination of new values for the levels of ammonia and nitrous oxidesdownstream of the selective catalytic reduction system via the modeluntil the difference is zero; when the difference is zero, a value ofthe maximum mass stored in the selective catalytic reduction system isemitted. 18: The method as claimed in claim 16, wherein if the valuedetermined for the maximum mass of ammonia stored in the selectivecatalytic reduction system is less than a threshold, a fault signalassuming a first value is emitted for the selective catalytic reductionsystem, while it assumes a second value if this is not the case. 19: Themethod as claimed in claim 17, wherein the levels of ammonia and nitrousoxides downstream of the selective catalytic reduction system areestimated via a model, as a function of the ratio between the quantityof nitrogen monoxide and nitrogen dioxide downstream of the exhaustmanifold, the temperature upstream of the selective catalytic reductionsystem, the mass of urea injected upstream of the selective catalyticreduction system, the maximum mass of ammonia stored in the selectivecatalytic reduction system, exhaust gas flow, the level of ammoniaupstream of the selective catalytic reduction system, and the level ofnitrous oxides upstream of the selective catalytic reduction system. 20:The method as claimed in claim 19, wherein the ratio between thenitrogen monoxide and the nitrogen dioxide downstream of the exhaustmanifold is determined as a function of the temperature upstream of theassembly of the particulate filter and the catalyst, exhaust gas flowfrom the internal combustion engine, and the level of nitrous oxidesdownstream of the exhaust manifold.